1. Field of the Invention
The present invention relates to an electric power steering device of an automobile and to a joint to be used in the electric power steering device.
2. Description of the Related Art
In the electric power steering device, a steering assisting force is obtained by an electric motor, the vibrations of which are transmitted through a steering shaft to the compartment of the automobile thereby to cause noises. These noises are serious especially in a hybrid car or an electric car, which intrinsically has a quiet compartment.
In an ordinary construction, the rotary shaft of the electric motor is so rigidly connected to a worm shaft by a serration joint or the like as to rotate together, and is connected in a torque transmittable manner to the steering shaft through the worm shaft and a worm wheel.
In order to prevent the steering assisting force of the electric motor from being unnecessarily fluctuated by a kickback applied from the road surface through the wheels to the steering shaft, there has been provided an electric power steering device (e.g., Patent Publication 1), in which a torque transmitting elastic member is interposed between the rotary shaft of the electric motor and one end of the worm shaft.
[Patent Publication 1]
JP-A-2002-145083
Here, the aforementioned steering assisting force of the electric motor is generated on the basis of a detection signal from a torque sensor for detecting the torsion of a torsion bar leading to the steering shaft.
Therefore, the steering assisting force is not generated in case the steering member such as a steering wheel is manipulated (or steered) by such an extremely small steering angle as hardly steers the steering shaft.
In this case, therefore, the steering wheel is steered while dragging (or turning as a load) the rotary shaft of the inactive electric motor. As a result, the starting torque at the time of starting the steering operation of the steering member thereby to deteriorate the steering feeling.
This defect likewise occurs in case the rotary shaft of the electric motor and the worm shaft are rigidly connected through the serration joint or elastically connected through the elastic member, as in Patent Publication 1.
This is because, even in the electric power steering device of Patent Publication 1, it is necessary for transmitting the torque to use an elastic member having considerably hard characteristics, which is hardly deformed at a small steering angle.